1. Field of the Invention
The present invention relates to a method of producing an optical fiber preform and an optical fiber, each having a plurality of holes extending in the axial direction.
2. Description of the Related Art
A typical optical fiber made of silica glass includes a core having a refractive index increased by, for example, being doped with germanium and also includes a cladding that surrounds the core and has a refractive index less than that of the core. Due to the effect of the total reflection of light that occurs at the boundary surface between the cladding and the core, light passes through within the core. Conventionally, the practicable relative refractive-index difference between the core and the cladding is no more than about 3% to 4%.
In contrast, recently, an optical fiber has been reported that has a relative refractive-index difference greater than that of such a conventional optical fiber (see, for example, Japanese Patent Application Laid-open No. H10-95628). It is reported in Japanese Patent Application Laid-open No. H10-95628 that, by forming, in the cladding, a plurality of holes extending in the longitudinal direction, the average refractive index of the cladding is largely decreased. In other words, such an optical fiber having holes has an effective relative refractive-index difference between the core and the cladding much greater than that of the conventional optical fiber.
Such an optical fiber having holes is produced by producing an optical fiber preform having holes and then heating and drawing it. Typical methods of forming holes on an optical fiber preform include a method of boring holes at predetermined positions on a solid glass preform by using a drill (see, for example, Japanese Patent Application Laid-open No. 2002-145634), a method of binding together a plurality of glass tubes and glass rods and then fusing the outer surfaces of the glass tubes and the glass rods together by heat in such a manner that the holes of the glass tubes remained (see, for example, Japanese Patent Application Laid-open No. H10-95628), and so on.
For such an optical fiber having holes, for the purpose of achieving desirable characteristics, it is preferable to have holes that are not deformed and are uniform over the entire length of the optical fiber in the longitudinal direction.
A method of producing an optical fiber preform with suppressed deformation of holes has been proposed that involves depositing glass particles on the outer circumference of a glass preform having a plurality of holes extending in the longitudinal direction, thereby forming a porous glass preform, and then sintering the porous glass preform, thereby producing an optical fiber preform having the holes extending in the longitudinal direction (see, Japanese Patent Application Laid-open No. 2004-244260).